Automotive brakes are of two general classes, namely hydraulic fluid operated and air pressure operated. Air pressure operated brakes have found wide application in heavy vehicles due to the convenience of transmitting compressed air from the point of control to the point of application. One popular type of air-pressure operated brakes, particularly on trucks, tractors and semi-trailers, is a cam actuated brake in which a push rod from an air actuating chamber mechanically displaces a cam through a slack adjuster which, in turn, amplifies the force and transmits it to the brake shoes. Another well-known type of air pressure operated brakes is a wedge actuated brake.
One of the problems encountered in air brakes is the provision of safe parking brakes when the air pressure had dissipated after engine shutdown or upon separation of the semi-trailer from the tractor. A widely used solution to the parking brake problem is the provision of a spring-engaged, air-released parking brake integrated with the air actuated service brake. In this system, release of the spring-actuated parking brakes is impossible until air pressure is available to perform it. Thus, the condition of a runaway vehicle completely lacking brakes is avoided.
Air brakes are necessarily relatively expensive compared to hydraulic brakes due to the need for air compressors, hoses and other accessories such as valves, relays and controls. In large vehicles, however, manual pedal input alone is insufficient to generate the magnitude of hydraulic fluid pressure needed to adequately actuate the brakes. Several types of boosted hydraulic brakes are employed which augment the manual input using air or fluid pressure or vacuum controlled by the manual input to achieve the required hydraulic pressure. One type of boosted hydraulic brake system employs the hydraulic pressure available from a power steering system to boost the operator's mechanical input force to the higher levels necessary to apply the brakes of truck vehicles. In this type of system, the power steering fluid is circulated through a booster which, when actuated by the operator's control substantially increases the fluid pressure in the brake fluid lines as compared to manual input alone. As in air brake systems, the mechanical output of the brake cylinder can be applied to cam operated or wedge operated brakes. Also as in the air brakes, the problem of providing a parking brake remains. In another type of boosted hydraulic brake system, hydraulic pressure of hydraulic transmission fluid from an automatic transmission fluid is used to boost the operator's mechanical input force to apply the service brakes and to release the parking brakes.
A spring-applied, fluid-release parking brake, similar in principle to the parking brake employed in air pressure systems is combined with the hydraulic service brakes to economically and compactly provide parking brakes in a cam operated hydraulically actuated brake system. The parking brake actuator and the service brake cylinder operate through a common linkage to actuate the cam operated brakes.
Hydraulic fluid is employed in the brake cylinder whereas power steering fluid is employed in the parking brake. These two materials are incompatible with each other and must be maintained separate from each other.
In both air and hydraulic systems, the problem of brake compounding can occur. Brake compounding is defined as the condition under which the force applied by the parking brakes is added to the force applied by the service brakes to the linkage, slack adjusters, drums and brakes. This can easily occur in either system when the service brakes are applied while in parking brakes are engaged. Compounding of forces in this manner could cause overstressing of brake components and brake drums. In air systems, anti-compounding is employed using a valve which transfers control of the brakes from one source of air pressure to another and thus avoids the additive application of force to the brakes. A simple two-way check valve is commonly employed for this purpose. A simple transfer mechanism such as a two-way check valve is not used in hydraulic brake systems because of the incompatibility of the hydraulic brake fluid with the other dissimilar fluid.